The Pentagon on Monday shifted its strategy to focus on the threat of Islamist militants and the spread of hi-tech missiles, warning that US military power faced new limits and constraints.

In a long-term strategy document, the Defense Department tossed out a doctrine entrenched for decades that the American military should be prepared to fight two wars at the same time against conventional armies.

Instead, the military must prepare for a range of threats in an "uncertain security landscape" where extremists or "non-state actors" are gaining access to missile technology and trying to secure weapons of mass destruction, the Quadrennial Defense Review said.

New weaponry, new tactics and new enemies had overtaken "the familiar contingencies that dominated US planning after the Cold War," Defense Secretary Robert Gates told reporters.

"The department's leadership now recognizes that we must prepare for a much broader range of security challenges on the horizon," said Gates as he presented the strategy review and the 2011 defense budget.

The review described a world where US warships, planes and satellites will face unprecedented threats from a range of missiles, with Washington's global network of regional air bases, ports and command centers increasingly at risk.

As missiles and other weaponry become more accessible, "US forces deployed forward will no longer enjoy the relative sanctuary that they have had in conflicts since the end of the Cold War," the report said.

As a result, the US military would need to bolster defenses at key bases against possible attack and take other measures to counter the new "anti-access" threats, it said.

As examples, the review cited how Lebanon's Shiite militia Hezbollah had acquired unmanned aircraft and portable air defense systems, while Iran had built up a ballistic missile arsenal and a fleet of small attack vessels to "swarm" US and allied navy ships.

The Pentagon, for the first time, identified global warming as a potential trigger of instability and urged the military to renew efforts to reduce its dependence on oil.

The review declared winning "today's wars" as the military's top priority, citing Afghanistan, Iraq and other unnamed countries where US forces could help to "dismantle terrorist networks."

The Pentagon called for more investment in aerial drones, special operations forces and helicopters, which have proved vital in the Afghan war and which all receive a funding boost in President Barack Obama's proposed defense budget.

Amid reports of US special forces working closely with Yemeni troops to target Al-Qaeda operatives, the review suggested a preference for helping other armies' take on militants rather than deploying large numbers of US forces.

"While the United States remains the most powerful actor, it must increasingly cooperate with key allies and partners to build and sustain peace and security," it said.

Gates said, "building the security capacity of partners has emerged as a key capability for this department."

That approach, "reduces the need for direct US military intervention, with all of its attendant political, financial and human costs," he said.

Both the strategy document and budget appeared heavily influenced by the wars in Iraq and Afghanistan, noting the stress placed on soldiers' families and urging a slower pace for deployments.

In keeping with the military's plans to set up a cyber command, the review identified cyberspace as a crucial new battlefield.

The department, which operates more than 15,000 computer networks across 4,000 military bases, faces daunting challenges but must train a cadre of experts and change how its staff views information technology, the review said.